Series gap lightning arrester with arc extinguishing chambers

ABSTRACT

This disclosure relates to an overvoltage spark gap protective apparatus having a series of arc gaps each within a spark gap chamber in combination with an outer coil to elongate the arcs. Each chamber houses a pair of spaced electrodes disposed nonsymmetrically therein to define an arc extension or elongating chamber to the one side of the electrodes. The walls of the confining chamber are formed of a semiconductor with a rough surface and are contoured to define a decreasing cross section with terminates in a narrow sidewall in the outer edge of the arc-elongating chamber. The outer end portion of the confining chamber, and particularly at the narrow sidewall, is connected by suitable vent passageway means to the exterior of the apparatus. The electrodes are mounted within an arcuate wall and secured thereto by an anchor lug along which the arc root moves from the electrode. A single wire magnetic coil is wound concentrically about the spark gap chamber and is connected in series with the protective arc gap electrodes. The coil serves to elongate the arc into the confining chamber and outwardly to the narrow sidewall.

United States Patent [72] inventors l [22] Filed [45] Patented [73] Assignee Appl. No.

[54] SERIES GAP LIGHTNING ARRFSTER WITH ARC EXTINGUISHING CHAMBERS 25 Claims, 14 Drawing 5.

[52] US. Cl 315/36, 313/156, 313/161, 315/347, 317/74 [51] Int. Cl H0lt1/04, H01t 5/02, 1102b 1/04 [50] FieldotSmrch 313/153, 161; 315/36, 347; 317/73, 74

[56] ReferencesCited UNITED STATES PATENTS 2,566,895 9/1951 Kalb 315/347 2,728,016 12/1955 Crouch et al. 317/73X 2,805,355 9/1957 Snell, Jr 317/74X 2,825,008 2/1958 Kalb 317/74X 3,151,273 9/1964 Stetson et al. 315/36X 3,248,600 4/1966 Sankey et al 3,443,149 5/1969 l-lazen Primary Examiner-Roy Lake Assistant Examiner-C. R. Campbell Attomey-Andrus, Sceales, Starke & Sawall semiconductor with a rough surface and are contoured to define a decreasing cross section with tenninates in a narrow sidewall in the outer edge of the arc-elongating chamber. The outer end portion of the confining chamber, and particularly at the narrow sidewall, is connected by suitable vent eway means to the exterior of the apparatus. The electrodes are mounted within an arcuate wall and secured thereto by an anchor lug along which the arc root moves from the electrode. A single wire magnetic coil is wound concentrically about the spark gap chamber and is connected in series with the protective arc gap electrodes. The coil serves to elongate the are into the confining chamber and outwardly to the narrow sidewall.

PATENTEUHAY den 3577.032

SHEET 1 0F 2 1 LIGHTNING WITH ARC "EXTINGUISHI NGCHAMBERSQ I This invention relates toan overvol't'age gap protective apparatus and'particulariy to, a voltage arrestor apparatus for a power line system and functioning to provide protection against surge or transient voltage conditions.

Power distribution systems are generally of a high voltage variety. External overvoltages can be caused by lightning and internal overvoltages can be caused by transients switching phenomena and the like. To design the overall system to accommodate such high transient voltage conditions'has therefore required expensive insulation and special design. The power systems companies also employ special protective devices, commonly identified as voltage arrestors, suitably interconnected to the system to absorb the transient voltage condin'ons. Generally, such arrestor devices employ some form of a spark gap device interconnected in a bypass circuit, along or in combination with special discharge resistors and the like. Generally satisfactory devices are available. Our devices generally involve heavy currents which impress severe duty requirements on the resistance elements and generally in many cases the eomtruction is such thatthe follow current may rise to a level to result in tripping of ground fault relays, blowing of fuses and the like. In the'construction of spark gap type devices electromagnetic means have been interconnected to apply a deflecting force upon the arc and cause it to elongate and to move from the spark over points on the electrodes. The extension of the arc length tends to increase the voltage of the arc and thereby increase the effectiveness and characteristic of absorbing energy. Generally, however, where ssruss electromagnetic coils have been employed they have been.

directly subjected to the high voltages with a resulting requirement of careful and expensive design to assure the integrity of the driving coils during the operation. Although permanent magnets have been employed, the magnetic intensity may be affected by the high surge current and not provide as effective repeatable operation as electromagnetic coil units. The electromagnetic device connected in the surge circuit has the additional advantage of providing a blasting effect or force which increases linearly with the arc current.

The present invention is particularly directed to an overvoltage spark gap protective apparatus having an improved enclosure for the spark gap elements which in combination with a blast control (kfines a highly reliable gap structure having a long life and consistent operation during both a duty cycle and'spark-over. The structure of the present invention into the arc elongating chamber.'A suitable venting system is provided to prevent establishment of pressures within the chamber which would interfere with the movement of the arc. Further, the wall of the chamber is defined by walls formed of a semiconductor which provides a limited or controlled conduction and provides inner walls which are nontracking and nongas producing. The field in accordance with a particularly novel aspect of the present invention is provided by a special electromagnetic coil which encircles the chamber and is interconnected in circuit with the spark gap electrodes to establish an electromagnetic blasting force which causes an are established between the electrodes to lengthen into the are 7 v elongating chamber. The force in addition to causing the elonhas an exceptionally high current capacity with inherent current limiting. The coil construction is in accordance with one aspect of this invention, such that under most applications, the necessity for surge coil protection is eliminated. The spark gap structure rapidly recovers to the blocking state and is essentially unaffected by atmospheric conditions such as moisture. The structure is relatively of a simple configuration with a consequent minimal cost and all components should have a relatively long service life. The characteristic provides an improved arrestor action which minimizes the follow current, which minimizes the resulting duty cycle on the valve blocks which might be employed minimizes the needles and undesired tripping of ground fault relays, the blowing of fuses and the like.

Generally in accordance with the one aspect of the present invention, the spark gap unit includes an enclosing chamber having electrodes disposed nonsymmetrically with respect to the chamber to define an arc extension or elongating chamber to the one side of the electrodes. The walls of the confining chamber or enclosure are contoured to define agenerally decreasing chamber cross, section in moving from the electrode chamber to the laterally outer edge of the arc elongating chamber. The are is elongated into the latter chamber and contacts the chamber walls tocoolthe arc plasma and thereby further increase the voltage drop across the arc.

in accordance with another significant aspect of the present invention, an electromagnetic coil is connected in circuit with the arc gap and serves to accentuate the movement of the are gation of the arc causes the cathode and anode spots on the electrode to rapidly move away from each other and drives the are into engagement with the opposite walls of the housing. Rapid movement of the cathode and anode spots minimizes or prevents electrode erosion. The inner walls of the chamber are preferably formed with a rough surface which tends to break up the arc and increase the voltage absorbing characteristic. The chamber walls further have a high thermal conductivity with arelatively low expansion. This minimizes the-possibility of breakage of the plates as a result of the thermal shock resulting when the arc strikes the walls.

The coil unit is constructed with a relatively low inductance to eliminate the necessity of special coil protection while maintaining sufficient strength to elongate the are within the insulating housing. Generally, the impedance of the power system circuit in combination with the gap structure is sufficient to limit the magnitude of the fault current to prevent destruction of the electrode. If necessary, of course, additional impedances may be connected with the coils and the spark gap.

Further, the tapered construction of the arc elongating cavity is preferably such that the outer edge includes a slight spacement and does not come to a definite fine point or edge. The particular spacing of the chamber may vary with the particular shape and taper of the cavity.

The electrodes are formed essentially of a nonmagnetic and nonferrous material. For example, a brass has been found to provide unusually satisfactory electrodes. Further, although the chambers are preferably sealed except for the venting, under'certairi applications they can be operated in an unsealed condition.

In accordance with another aspect of the present invention, are anchors are secured to the electrodes and extend from the electrode to the entrance portion of the arc extending chamber. The roots of the are are driven outwardly from the electrode along the anchors to the wall of the housing and thus prevent heavy electrode burning.

In a preferred construction of the present invention, the chamber is formed as an annular member with opposed contoured walls including a generally planar base portion defining an electrode housing chamber immediately adjacent one wall portion of the chambenSpaced slightly outwardly thereof, the top and bottom walls are tapered toward each other to the opposite end of the chamber to define the arch elongating chamber. The chamber is provided with a single vent to the back. side of the electrode chamber and a plurality of smaller vents in the narrow end of the arc elongation chamber. Alternatively, a single lateral extending vent may be provided extending through the housing immediately adjacent and in.

communication with the terminal or narrow portion of the are extending chamber. In either structure, a pressure differential is built up tending to move the arc outwardly.

In a preferred construction of this invention, the coil unit is connected in series with a pair of spark gap units, the housing assembly includes a central plate or housing member having similar electrodes secured with terminal elements extending in opposite directions. The end caps or housings are secured to the middle housing and each includes a single electrode cooperating with that of the middle plate to define the spark gap. Each end electrode includes an outwardly extending ter' minal means for connecting the spark gap unit for circuit. The

opposing-walls of these several plates or housings elements are contoured to define the electrode chamber and the generally tapered or horn gapped type elongating chamber extending laterally therefrom. The blast coil unit is wound about the central plate with the opposite ends interconnected respectively to the electrodes of the central plate. The current is through the electrode gap in one chamber through the coil and then through the electrode gap in the opposite chamber to provide a series circuit connection of the arc gaps and the winding. The coil is preferably formed as a single layer coil of a few 'tums which are tightly wrapped around the housing and secured with a suitably insulating adhesive covering to physically attach the coil directly to the assembly.

An alternative parallel circuit connection may be employed, thus, a series of three stacked gaps may be fonned from similar opposite end caps with a pair of intermediate stacked plates. EAch of the intermediate plate units includes an electrode assembly with a common contact extending therethrough and terminating in the opposite ends in electrodes. The stacked assembly properly locates the electrodes to provide a series circuit through the three gaps. A coil is wound about one or more of the arc gap units with the leads interconnected to the electrodes located in central arc gap chamber. The coil is then connected in parallel with the central gap and in series with the opposite end gaps.

The present invention provides a highly improved arc gap structure providing a long service life with continuous and reliable repeatable operation over the life of the assembly.

The drawings furnished herewith illustrate the best mode presently contemplated by the inventors for carrying out the invention and disclose the above advantages and features as well as others which will be readily understood from the following description of the drawings.

In the drawings:

FIG. I is a side elevational view of a lightning arrester with parts broken away to show inner details of construction;

FIG. 2 is a view of the spark gap device shown in FIG. 1 with parts broken away;

FIG. 3 is a top plan view of FIG. 2;

FIG. 4 is a horizontal section taken generally on FIG. 2;

FIG. 5 is a similar horizontal section taken generally on line line 4-4 of I as ofFIG. 2;

- FIG. 6 is a view taken generally on line 6-6 of FIG. 2;

FIG. 7 is a fragmentary developed view talren generally on line 7-7 of FIG. 3;

FIG. 8 is a'fragmentary vertical section taken generally on line 8-8 of FIG. 3;

FIG. 9 is a side elevational view illustrating an alternative construction in accordance with an arrestor constructed in accordance with the present invention;

FIG. It) is a top plan view of the arrestor shown in FIG. 9;

FIG. II is a horizontal section taken generally on line 1 I41 of FIG. 9;

FIG. 12 is a framnentary vertical section taken generally on line 12-12 of FIG. ll;

FIG. 13 is a generally vertical section taken on line l3-ll3 of FIG. It); and

FIG. 14 is a vertical section taken generally on line 34-14 of FIG. 9.

Referring to the drawings and particularly to FIG. 1, the illustrated embodiment of the invention includes a lightning arrester ll interconnected between a power line 2 and a ground 3. Thearrestor generally includes an outer tubular housing 4 which in accordance with known construction is formed of porcelain or other suitable insulating material. Suitable end cap terminals 5 are similarly sealed to the opposite ends of the housing 4 and simultaneously function as a closure, a physical roster, for example, similar to the assembly shown in US. Pat. No. 3,242,376 to I". ,I. Schultz. The end caps 5 may be tor-med a any suitable y condocdve metal and am my hersupport and an electrical terminal means for the lightning arrney to the housing to enclose the internal components, and provide an eiec trical connection between the power system and such components. In the illustrated embodirnent of the invention, blocir resistors 6 and 7 are di adjaeent the mpective end caps 5 with an intermediate or centraily located are or spark gap wait 8 interposed in stacked relation therebetween. Generally, the resistors 6 and 7 may be suile voltage sensitive resistors formed of a suitable material having a high es in the absence of current flow therethrough. An arc drive coil 9 is telescoped about the arc gap unit and interoonn in circuit therewith as more fully developed hereafter. The are gap unit 8 normally maintains an open drcuit condition through the arrestor 1. When the gap unit 8 breaths down, as a result oi a surge or a transient voltage of a sell level, current flows through the arrestor l and the reflective more of the resistors 6 and 7 rapidly decreases such that the current correspon 1 increases and the surge or transient voltage is iimi to a reasonable value.

As the transient voltage decreases, the resistance of the resistors 6 and 7 increases until such time as normal line voltage again appears across the arrester l. The gap unit 8 resets and the current flow through the arrester essentially terminates.

The present invention is particularly directed to the construction of the arc gap unit 8, one embodiment of which is shown in FIG. 1 through 8, inclusive.

Referring particularly to FIGS. I and 2, the illustrated spark gap unit 8 generally includes a bottom gap electrode housing Ill, a first middle gap housing II, a second middle gap housing l2 and a top electrode housing 13 disposed as a stacked assembly. Generally, the top and bottom housings l0 and 13 and the two intermediate or middle plates 11 and 12 are similarly constructed. The mating faces of the housings are contoured to define a series of three are gap chambers within which gap electrodes are mounted, as shown in FIGS. 2-8 and described as follows.

The bottom electrode housing 10 is generally a cup-shaped member formed of a suitable semiconductor material. A preferred material is a silicon carbide interconnected by a suitable nonorganic bonding media; for example, as more fully disclosed in applicant's copending application entitled Spark Gap Apparatus, which was filed on July 9, 1968 with Ser. No. 743,365 and is assigned to the assignee of this application. The silicon carbide material not only provides an integral grading resistor as disclosed in the above application, but further provides additional physical characteristics uniquely applicable to the requirement of the present spark gap apparatus. The material has a high melting point and thermal conductivity in combination with excellent thermal stability and low thermal expansion coefficiency. The semiconductor and inorganic binding material provide nontracking material which is nongas producing in the presence of an arc. The limited conductivity required to provide the voltage grading as disclosed in the copending application will not in any way interfere with the desired protective functioning in the spark gap protective appara'tus of the present invention.

The housings may also be formed of a high talc and/or Wollastonite body, nonvitrified, maturing'in the range of cones 04- -S and containing only a small amount of glass of glassy phase. A particular body with which considerable success has been obtained consists of 72 percent Wollastonite, 22 percent Clay and 6 percent Kaolin fired at 1,040C. The latter material, however,.is a substantially insulating material and thus does not provide the desirable voltage grading characteristics ob tained with the semiconductor materials.

The cup-shaped housing 10 has the outer edge or lip mating with a corresponding edge or lip of the adjacent intermediate housing ill with an exterior annular recess 14 at the interface. A suitable sealing compound such as silastic gasket 15 fills the recess and seals the arc chamber 16 defined by the internally contoured recesses of the housings l0 and 11.

Generally, the arc chamber 16 includes a deep wall portion 17 to the one side of the unit defined by parallel planar bases 18 of the housings l0 and ii. A pair of electrodes 19 and 20 are supported in a laterally spaced relation with the electrode 19 carried by the bottom housing and the electrode carried by the first intermediate housing 117 The contoured bases of the housings l0 and II included tapered bases 21 which extend laterally and outwardly from the deep wall portion l8 to the opposite end or side of the arc chamber I7 to define a generally wedge-shaped space which in turn constitutes an arc-elongating chamber extending outwardly from the electrodes I9 and 20.

The electrode 19 which is supported by the bottom housing 10 is secured to one end of a supporting rivet 22 which ex tends through the base I8 of the housing 10. The illustrated electrode 19 is in the form of a flat washer and is clamped by the head of rivet 22 against an integral boss 24 in the housing which locates the outer face of the washer with respect to the plane of the lip of the cup housing 10. A contact plate 25 is secured to the exterior face of the housing I0 and interconnected through to the outer end of the rivet 22, as most clearly illustrated in FIG. 2 and 8. The bottom resistor 6 is therefore connected electrically to the first electrode 19 through the contact plate 25 and the rivet 22.

In the illustrated embodiment of the invention, a preionizer plate or disc 26 which may be formed of a material similar to that of the housing, is shown secured in physical and electrical The disc 26 may be attached to the electrode 19 and by a suitable adhesive, for example, a conducting epoxy.

As most clearly shownin FIG. 4, the housing I0 is formed with an offset sidewall 27 defining an inward projection extending about the portion of the electrode 19 adjacent the initial portion of the tapered base'2l. An electrode anchor 28, in the form of a conducting member or bar, is secured to the electrode I9 and extends outwardly along the adjacent projecting wall 27. i

The intermediate housing 11 is generally symmetrically formed about a cenualhorizontal plane as shown in FIG. 7, and includes a top reces 29 and a bottom recess 30 cooperating with the adjacent members 10 and 12 to form are chambers. The electrode 20 is disposed in the bottom recess and supported by a rivet 31 extending upwardly through a bosses portion of the housing II to locate electrode 20 with its lower plane essentially in the lower plane of the housing II. The electrode 20 includes a preionizer 32 and an anchor lug 33 interconnected to the electrode 20 and to an offset wall 34 of the housing in generally the same manner as the electrode unit 19.

In the operation of the system, the preselected voltage appearing across the electrodes 19 and 20 result in a generation of an are 35. Referring particularly to FIG. 4, as a result of the action of the coil unit 9, as more fully developed hereinafter the are 35 moves outwardly as an elongated are into the horn gap portion defined by the tapered walls 21 and with the roots of the are 35 moving outwardly along the electrodes 19 and 20 then along the anchored lugs 28 and 33. The elongated arc 35 moves into engagement with taper walls and into the narrow confinement of the horn portion.

In the illustrated embodiment of the invention, the movement of the are 35, and the resulting voltage characteristic of the arc, is improved by the provision of a laterally extending vent opening 36 aligned with the outer most end of the arc chamber. The vent opening 36 prevent buildup of pressure within the arc chamber which could interfere with elongation of the are 35. Further, the housings I0 and II are formed with the inner surface of a rough texture as diagrammatically shown at 37 in FIGS. 4 and 8 to break up the arc 35 and thereby further increases its voltage drop characteristic.

The housings I0 and II are interconnected in any suitable manner and preferably provided with a conductive film 38 such as a silver painting covering a portion of the mating faces to minimize radio interference characteristics under arcing conditions. The housings II and 12 form a second arc chamber 39 generally corresponding to the lower arc chamber 16 but angularly displaced, as shown in FIGS. 4 and 5,

An electrode 40 is interconnected to the opposite of the rivet 31 and supported within the arc chamber 39. The electrode generally corresponds to that electrode 19 and 20 and is secured to rivet 31 with its outer or upper face essentially in the plane of the top edge of the housing II. A preionizer disc M and an anchor 42 are similarly secured to the electrode 40 and housing 11.

Referring particularly to FIGS. 3 through 5, in the illustrated embodiment of the invention, the several housings are similarly constructed and angularly oriented to dispose the electrode units in the top and bottom are chambers generally to the opposite sides of the electrodes in the central arc chamber. Thus, the arc chamber 39 is angularly oriented with respect to the bottom chamber I6 to shift the electrode pair within the chamber 39 to the angle corresponding to the spacing of the electrodes to thus properly locate the electrodes as shown in the drawings.

The housings II and I2 are provided with corresponding recesses adjacent the narrow end of chamber 39 to define a vent 43 for the intermediate arc chamber 39 corresponding to vent 36.

Further, as shown most clearly in FIG. 5 the upper edge of the intermediate housing 11 is provided with a small recess 44 extending through the anchor wall 45. A lead 46 for the coil 9 is secured to the electrode 40 and extends outwardly through such recess and the silastic gasket 15 to form one power connection to the coil 9. i

The housing 12 which is secured in stacked relation to the housing 11 is generally formed in a manner identically to that of the housing 11, with the lower surface contoured to define arch chamber 39. A pair of electrodes 47 and 48 are interconnected to a common terminal or rivet 49. The electrode 47 is constructed and mounted in the same manner as the other electrodes and is disposed in laterally spaced relation to the electrode 40. The lower surface of the housing 12 is provided with a coil lead recess 50 aligned with the electrode 47, The opposite coil lead 51 of the coil 9 is interconnected to the electrode 47 and extends outwardly through the recess 50 to connect the coil 9 in parallel with the electrodes 40 and 47.

Similarly, the upper face of the plate or housing 12 is con- 4 toured to cooperate with a contoured mating surface of the uppermost housing 13 to define an upper arc chamber 52 within which the electrode 48 is disposed.

The top housing 13 is generally constructed in accordance with the illustration of the bottom housing 10. An electrode 53 is secured within the housing by a contact and support rivet 54 which extends upwardly through and is interconnected to a top contact plate 55. In the assembly of FIG. I, plate 55 provides the top electrical connection to valve block 7.

The coil as illustrated in FIG. 1, extends throughout the depth of the several arc chambers 16, 39, and 52 preferably provided with an outer coil insulation 56. The field of the coil 9 is therefore coupled to the several arc chambers and in particular to the arcs 35 established between the several electrodes.

The operation of the illustrated embodiment of the invention is briefly summarized as follows; the gaps between the several electrodes in the arc chambers normally maintain an open circuit condition through the arrestor I. If a surge voltage is applied acros the arrestor I, the space between the electrodes of the serially connected spark gaps is ionized by the high electrical stress between the preionizers and a spark or arc 35 is established within the several chambers with a cathode and anode spot on the respective electrodes within the chambers. A discharge is established essentially immediately between the metal electrodes whereby the surge energy passes through the serially connected gaps. Arcs 35 initially are established directly across the electrodes, for exam ple, as shown in FIG. 4, the portion of the surge voltage apt pearing across the electrodes 40 and 47 in the central chamber 39 is also applied directly to the electromagnetic coil 9. The are has a normal tendency to move away from the electrodes and this tendency is strongly accentuated by the magnetic field of the follow current flowing through the coil 9. The anode and cathode spots move rapidly away from each other along the circumference of the appropriate electrodes. The electromagnetic force and the forces generated as a result of the venting of each chamber causes the arcto extend outwardly into the horn gap portion of each cavity and in particular into the continuously smaller and confining portion. As they move outwardly into the confining portion, the hot gases forming the plasma come into intimate contact with the roughened surfaces of the housings 10-13. The high thermal conductivity rapidly cools the gases. This is further augmented by the rough surfaces. The total effect of the movement and cooling is to increase the impedance of the current path and thereby to limit the current and increase the voltage drop in each are 35.

The sealing and controlled venting provided by the interconnection of the housings and the sealing gasket and the vents 36 and 43 provide a controlled discharge for the hot ionized gases and vapors generated within the chamber and insure that the gases and vapor ejected are thoroughly deionized.

The resistance of the gap in the central chamber 39 rapidly increases and within a matter of microseconds exceeds the resistance of the coil 9. The power frequency follower current which follows through the circuit will flow through the coil 9 in preference to the resistance path through the gap between electrodes 40 and 47 in the central chamber 39.

The power frequency current therefore does not pass through the intermediate arc gap but through the coil 19 in series with the top and bottom are gaps provided by the several electrodes in the top and bottom are chambers 16 and 52.

The above action provides a spark gap apparatus which will limit the follow current to a magnitude which will eliminate the needless tripping of ground fault relays, the blowing of fuses and like encountered in previous are devices. The current limiting feature also limits the load or duty of the material in the resistors 6 and 7, when employed, and thereby extends the service life of the unit. The very low surge impedance of the cunent limiting gap structure will also reduce the voltage drop across the resistor below that encountered in systems employing high impedance coils.

Upon full extension of the arc columns 35, the arc roots have moved along the anchor lugs 33 and the like and remain fixed to the tip of the anchor lugs. The final arc column 35 thus is a steady burning arc of a high voltage drop characteristic extending along the outer wall of the narrow confining portion of the arc chamber. The sum of the voltage drops across the several gaps and the coil 9 provides substantial and desirable current limiting features. The similar housings and electrode units provides a relatively simple and inexpensive construction particularly adapted to commercial production line methods. The several components and parts employed are also inexpensive and of readily available materials which further minimizes the initial cost.

The elongation and cooling of the plasma column causes the total are voltage to increase both by the simple lengthening procem and by increasing the impedance per unit by length of the arc column. The total are voltage created in this manner is a measure of the current limiting ability of the gap structure.

Applicant has found that the dimensional arrangement and spacing of the walls in the chamber will effect the particular desired action. In an actual construction, electrode spacing in a range of 0.5 to 4.0 millimeters were investigated. Generally, it was found in the particular application that a space of 2 millimeters produced a preferred arrangement. The height of the enclosed electrode chamber was varied from 0.l to 4.0 millimeters and a preferred height of 0.25 millimeters was arrived at, for at least part of the chamber. Diameter of electrodes were also found to generally be related to the diameters of the enclosed space. Although diameter ratios from 2-10 has been employed and operated satisfactorily, a ratio of 3-1 was found to provide a highly satisfactory ratio. Similarly, the thickness of the electrodes with relationship to the depth of the chamber will affect the ratio although a particular height limi ation was not noted. Although the coil 9 may be connected in parallel with a gap structure, it is not necessary and an alternative embodiment employing a direct series coil connection is illustrated in FIGS. 9-14 of the drawing.

The construction of the unit in FIGS. 9-14 generally includes a bottom housing 57, a single intermediate housing 58 and a top housing 59, interconnected in stacked relationship with a suitable sealing compound 60 disposed within recesses between the adjacent edges of the housings to provide sealed chambers. The mating faces of the housings are contoured to define a pair of sealed arc chambers 61 and 62 to the opposite sides of the middle housing 50.

A bottom contact plate 63 and an upper contact plate 64 are connected to the corresponding faces of the housings 57 and 59. A series coil 65 is wound about the middle housings 58 and connected in series with the arc gaps as presently described to provide a series coil circuit generally functioning to provide an improved spark gap apparatus as follows.

The several housings in a stacked structure may advantageou sly as previously noted, be formed of a semiconductive material. If desired, all of the housings except that incorporating the coil or coils may be of a semiconductor with the others formed of a nonvitrified material. Referring particu' larly to FIGS. 11-13, the bottom housing 57 includes a contoured cavity base 66 generally constructed in accordance with the cavities shown in the first embodiment. An electrode 67 corresponding to those heretofore disclosed is attached to a terminal rivet 68 extended through the base of the housing and interconnected to the contact plate 63. An ionizer disc 69 and anchor lug 70 are similarly secured to the electrode. The cavity 66, in the illustrated embodiment of the invention, is provided with a somewhat wider electrode portion extending for slightly less than one-half the diameter of the arc chamber 61 to the initial edge of the taper portion as shown in FIGS. 13 and 14.

An electrode 71 is supported by the middle housing 58 in laterally spaced relation to the electrode 67 and defines a spark gap within the arc chamber 61.

A small vent 72 extends radially outwardly of the electrode portion of the arc chamber 61 and is located generally centrally between the two electrodes 67 and 71. Three radial vents 73, 74, and 75 extend outwardly in the diametrically opposite side of the arch chamber 61 and thus outwardly from the narrowmost end of the arc confining portion of the arc chamber.

In operation the arc chamber 61 is similar to that of the top and bottom are chamber shown in the first embodiment and provides a similar interaction to provide a controlled surge voltage arc, not shown.

The middle or central housing 58 is generally formed symmetrically about a horizontal plane with top and bottom sur faces contoured to define the lower arc chamber 61 and a similar top are chamber 62.

The electrode 71 is secured in the electrode portion of the housing 58 and in particular is connected therein by a rivet 76, as most clearly shown in FIG. 13. The rivet 76 extends through the base portion of the housing and tenninates within a recess 77 formed in the opposite tapered wall of the top chamber cavity. A cover 78 is secured within the recess 77 with its outer surface in the plane of the tapered portion of the upper cavity or chamber 62. The sealing disc or cover 78 is preferably formed of the same semiconductor material as that of the housings 57-59 and is made to lie exactly flush with the surrounding tapered surface. The cover 78 can be cemented or otherwise physically supported in the opening. If an organic cement is employed to cement the cover within the opening, it should provide a very snug fit such that a very minimum amount of cement will be exposed to the action of the are as it moves through the arc-elongating chamber.

If an inorganic cement is employed, the fit of course can be considerably looser.

Alternatively, a mechanical support may be supplied. For

example, the cover 78 may be formed with an integral extension 79 which extends downwardly through the recess 77 and is interconnected to the rivet 76 to provide a mechanical support.

Referring particularly to FlGS. 11 and 14, the intermediate or middle housing 58 is provided with generally radial recesses 80 and 81 on the opposite faces and each extending outwardly through the inward projection of the walls adjacent the electrode 71 and an electrode 82 located within cavity 62. One coil lead 83 is connected to the bottom electrode 71 and extends outwardly through the recess 80. The opposite coil lead 84 is interconnected to the electrode 82 top electrode unit of the middle housing and extends outwardly through its recess 81 and is connected to the opposite end of the coil 58.

The illustrated coil 65 is a single layer tightly wrapped around the intermediate housing 58. The coil 65 is attached to the housing by a high strength insulating adhesive such as an epoxy cement cover 85: This simultaneously provides a physical support for and insulation of the coil.

The top or upper housing 59 generally corresponds in construction to that of the lower housing and defines the top wall of cavity 62. An electrode 86 is supported by a rivet 87 which passes upwardly through the base of the housing. The outer end of the rivet 81 is interconnected to the contact plate 64 to complete the circuit therethrough.

In the second embodiment of the invention, both the surge current and the follower current take the same conducting path through the assembly. Starting with the top contact plate 64, the current flows through the rivet 87 to the first electrode 86, across the gap to the second electrode 82 and then through the coil 65 to the other electrode 71 of the middle plate or housing, across the second gap to the lower electrode 67 and the rivet 68 to the bottom contact plate 63 to complete the path.

The series coil 65 is wound to simultaneously force the anode and cathode spots along the several electrodes and over the anchor lugs regardless of the direction of the flow of the current.

As in the first embodiment the movement of the anode and cathode spots minimizes the electrode burning and also results in an effective lengthening of the arcs. This action plus the effect of the pressure differential due to the several vents or parts cause the highly ionized gases in the arc plasma to increase in length and to be driven into the tapered and confined section of the cavity. They are moved outwardly into intimate contact with the roughened surfaces of the chamber with the resulting cooling and further increase in the impedance causing further limiting of the current.

Applicants have found that it is possible to operate a gap assembly such as shown on FIGS. 9-14 in a 3 kilovolt RMS circuit having an effective source impedance of approximately 3 ohms without the necessity of using other additional current limiting means or devices; for example, the illustrated valve elements of the FIG. 1. If the effective source impedance is less than 3 ohms per 3 kilovolts, some additional impedance is normally required.

The are voltages appearing across the assemblies can generally be maintained above l kilovolt for approximately the last 4 milliseconds of a 6 millisecond conduction period. This causes an early current zero and allows the gap stnicture to fully recover its dielectric strength before die next voltage crest is applied to the system.

The integral grading resistance provided by the housings constructed in accordance with applicants copending application also provide the uniform voltage distribution across the arcs. in the quiescence state, a current in the order of l to 4 rnilliamps may flow through the semiconductor housing. The majority of the voltage drop appears across the upper and lower gap housings and therefore across the upper and lower gaps.

The slightly rough surface textures of the cavity permits the ionized gases within the arc to move freely over the surface while producing minimal gas evolution from the surface. This action is particularly significant in that it permits the gaps to operate repeatedly with minimal deterioration.

Although the invention has been illustrated, employing electromagnetic coil units because of the distinct advantages thereof, within the broadest aspects of the invention, suitably shaped and oriented permanent magnets can be employed.

The illustrated embodiment of the inventions are given to clearly disclose the best modes and to clearly illustrate the various advantages and features of the optimum construction. Other suitable construction based on the teachings may be employed. For example, although circular housings and electrodes have been illustrated, other configurations can readily be constructed and provide suitable operation. Typical dimensional arrangement of a unit constructed in accordance with the second illustrated embodiment of the invention are as follows:

1. Housing Material-alpha silicon carbide ceramic 2. Outer Diameter of Housingsl00 millimeters 3. Diameter of Arc Chambers-8O mm.

4. Maximum Cavity Depth-35kmillimeters 5. Height Electrode Locating Boss3%millimeters 6. Electrodes-Brass 7. Electrode Spacing2l 4 millimeters 8. Electrode diameters 17% millimeters 9. Electrode Thicknessl %Cmillimeters 10. Anchors-2X2 l 6 millimeters, formed of bras and connectedtoelectrodebyasritablehighstrutgthandoonductive epoxy cement.

ll. Preionizers-Same as housings and cemented to electrodes with a high strength and conducting epoxy cement.

l2. Preionizer Diameters l9 /zmillimeters l 3. Preionizer Thicknessl millimeter The present invention thus provides a highly improved spark gap apparatus which can be employed for reliable and repeatable voltage surge protection over long service periods. The manufacturing cost is minimized and thus particularly makes the device suitable for commercial application.

We claim:

1. An overvoltage gap protective apparatus, comprising an enclosure means defining a spark gap chamber having an electrode portion and an arc-elongating portion extending outwardly from the electrode portion with a generally continuously reducing cross section to an outer temiinal portion, vent means in said enclosure means connected to said terminal portion of the spark gap chamber, spaced electrode means in the electrode portion of said chamber and having terminal means for connecting said electrode means in a circuit to be protected, said spaced electrode means having an are established therebetween in response to a selected applied voltage, and a magnetic means coupled to said enclosure means to elongate the are into the arc-elongating portion of said chamber.

2. The overvoltage protective apparatus of claim 1, wherein said magnetic means is an electromagnetic means connected in circuit with said electrode means.

3 The spark gap protective apparatus of claim 1, wherein said magnetic means is an electromagnetic means connected in series with said electrode means.

4. The spark gap protection apparatus of claim 1 having a second similar enclosure means and a second electrode means in said second enclosure means, and said magnetic means is a common electromagnetic means encircling both of said enclosure means and connected in series between said first and second electrode means.

5. The spark gap protective apparatus of claim 1, wherein said magnetic means is an electromagnetic coil of a single layer wound about the enclosure means and connected in series with the electrode means.

6. The overvoltage protective apparatus of claim 1, wherein said enclosure means includes a pair of stacked housings having complementing cavities defining said chamber, the bases of the cavities including parallel planar surfaces along corresponding sides thereof and having correspondingly tapered wall extending laterally from the planar surfaces and toward each other, a'ndvent means connected to the chamber at the outer'end portions of the tapered walls.

7. The overvoltage protective apparatus of claim 1, wherein said enclosure means includes a pair of stacked housings having complementing cavities defining said chamber, the bases of the cavities including parallel planar surfaces along corresponding sides thereof and having correspondingly tapered roughened walls extending laterally from the planar surfaces and toward each other, said stacked housings being formed of a semiconductor material a conductive adhesive between said complementing surfaces to interconnect the housings and provide a current path thereberween, a sealing means secured to the outer junction of the housings to seal said chamber, and said vent means being connected to the chamber at the outer end of the tapered roughened wall.

8. The overvoltage protective apparatus of claim 7, wherein the housings are formed of a nonvitrified material.

9. An overvoltage protective apparatus comprising an enclosure means including three stacked housing, said housings having complementing cavities defining a pair of arc chambers, the opposite faces of the intermediate housing having similar cavities each including a planar wall along one side thereof and having a tapered wall extending laterally from the planar wall, said cavities having the planar walls on diametrically opposite sides of the housing, the end housings being formed with similarly contoured cavities aligned with the adjacent cavity of intermediate housing to define a pair of spark gap chambers having an electrode portion and an arc elongating portion extending outwardly from the electrode portion with a generally continuously reducing cross section. electrode means including spaced electrodes having a rodlike terminal and support members and means securing an electrode to each of the planar walls with the terminal and support members extending therethrough to locate a pair of spaced electrodes in each arc chamber, the terminal members secured to the intermediate wall temiinating in a recess in the tapered wall, means secured within each of the recesses to seal the recesses and define a continuous uninterrupted tapered wall, and a magnetic means coupled to said enclosure means to elongate the arcs into the arc elongating portions of said chambers.

10. An overvoltage protective apparatus comprising enclo sure means including three stacked housings having complementing cavities defining a pair of arc chambers, the opposite faces of the intermediate housing having similar cavities each including a planar wall along one side thereof and having a tapered wall extending laterally from the planar wall, said cavities having the planar walls on diametrically opposite sides of the housing, the end housings being formed with similarly contoured cavities aligned with the adjacent cavity of the intermediate housing to define an electrode portion and an arc elongation portion extending outwardly from the electrode portion, electrode means including electrodes having a rodlike terminal and support members, and means securing an electrode to each of the planar walls with the terminal and support members extending therethrough to locate a pair of spaced electrodes in each arc chamber, said spaced electrodes having an are established therebetween in response to a selected applied voltage, and an electromagnetic coil means as a single coil tightly wound about the intermediate housing to elongate the arc into the arcelongating portions of said chambers, and an insulating adhesive covering said coil and attaching said coil to said intermediate housing.

11. The overvoltage protective apparatus of claim 10, wherein all housings are formed of serniconductive material.

12. The overvoltage protective apparatus of claim 10, wherein all housings are formed of nonvitrified material.

13. The overvoltage protective apparatus of claim I, wherein said enclosure means includes a plurality of stacked housings having complementing cavities defining arc chambers, the opposite faces of an intermediate housing having similar cavities each including a planar wall along one side thereof and having a tapered wall extending laterally from the planar wall, said cavities having the planar walls on diametrically opposite sides of the housing, the adjacent end housings having being formed with similarly contoured cavities aligned with the adjacent cavity of intermediate housing, means securing an electrode to each of the planar walls, and an electromagnetic coil means tightly wound in a single layer construction about the housings, and an insulating adhesive covering said coil means and attaching said coil means to said housings.

14. The overvoltage protective apparatus of claim 13, wherein the housings having the coil means wound thereon are of nonvitn'fied material and all other housings are of a semiconducting material.

15. The overvoltage protective apparatus of claim 1, wherein said enclosure means includes a pair of stacked housings having complementing cavities defining said are chamber, the bases of the cavities including parallel planar surfaces along corresponding sides thereof and having correspondingly tapered wall extending laterally from the planar surfaces and toward each other, and terminating in a circular sidewall of a selected minimum depth at the outermost ends of said tapered walls, a vent opening extending along a tangent line through the outermost ends of said tapered walls,

16. An overvoltage protective apparatus comprising an enclosure means defining a spark gap chamber having an electrode portion and an arc-elongating portion extending outwardly from the electrode portion with a generally continuously reducing cross section, spaced electrode means in the electrode'portion of said chamber and having terminal means for connecting said electrode means in a circuit to be protected, said spaced electrode means having an arc established therebetween in response to a selected applied voltage, said enclosure means including a second arc chamber having a second pair of electrode means and a third are chamber housing a third pair of electrode means, said three chambers being arranged in superimposed stacked relation, conducting means connecting one of said first pair of electrode means to one of said second pair and the second of said second pair to a first of said third pair to connect the electrode gaps in series, and electromagnetic means coupled to said enclosure means to elongate the arc into the arc elongating portion of said chambers and connected in parallel with the second pair of electrode means.

17. The overvoltage protective apparatus of claim 16, wherein each of said are chambers having a-corresponding contour including an electrode portion to one side of the chamber with tapered walls extending outwardly from the electrode portion to define a gap horn, said outer chambers being oppositely axially rotated with respect to the central chamber with the pair of electrode means in the central chamber aligned with one of each of the electrode means in each of said outer chambers.

18. The overvoltage protective apparatus of claim 1, wherein said enclosure means includes a pair of stacked housings having complementing cavities defining said chamber, the bases of the cavities including parallel planar surfaces along corresponding sides thereof and having corresponding tapered wall extending laterally from the planar surfaces and toward each other, said vent means being connected to the chamber, said electrode means having terminal members extending outwardly through said planar surfaces and supporting a platelike electrode within the chamber, the sidewall of each housing including an inner projection complementing the electrode adjacent the tapered walls, and conductive anchor members connected to the corresponding electrodes and extending outwardly along the corresponding projection to the sidewall of the chamber adjacent the tapered walls.

19. The overvoltage protective apparatus of claim 1, wherein said enclosure means includes a plurality of stacked housings with the mating surfaces having complementing contoured cavities having circular sidewalls and defining a series of arc chambers, the bases of the cavities including parallel planar surfaces along corresponding sides thereof defining platelike electrode within the corresponding chamber to locate a pair of electrodes in spaced relation within each electrode portion of said chambers, the sidewall of each housing including an inner projection complementing the electrode adjacent the tapered walls, conductive anchor members connected to the corresponding electrodes and extending outwardly along the-corresponding projection to the sidewall of the chamber adjacent the tapered walls, contact plates secured to the opposite ends of said stacked housing and connected to a terminal member extending outwardly from the immediately adjacent housing, said magnetic means including an electromagnetic coil encircling said housings and connected in series with at least one of said pair of electrodes.

20. The overvoltage protective apparatus of claim 19 having at least three similar arc chambers each having the pair of electrodes similarly located therein, chambers being angularly oriented with the electrodes in the outer chambers being angularly spaced in accordance with the spacing of each pair of electrodes, and wherein the electrodes in the central chamber are each aligned with one of the-electrodes in the outer chambers and connected thereto by a common terminal member, and said electromagnetic means is connected in parallel with wherein said enclosure means includes a pair of adjacent similar arc chambers oppositely angularly oriented to locate the electrodes to opposite sides of the enclosure means, and said electromagnetic coil is a single layer coil tightly wound about the central portion of the enclosure means between said adjacent similar arc chambers, and said coil being connected in series between said pairs of electrodes.

22. An overvoltage spark gap protective apparatus, comprising an enclosure means defining a spark gap chamber having an electrode portion and an arc elongating portion has a maximum depth at the electrode portion, and extends outwardly therefrom with a continuously reduced depth to a terminal portion having a selected depth to establish a continuv ous sidewall in said arc-elongating portion.

23 The overvoltage gap protective apparatus of claim 22 wherein said spark gap chamber includes vent means to discharge gas generated within said chamber, said vent means being located to discharge said gases in a di-ionized state.

24. The overvoltage gap protective apparatus of claim 22 wherein said enclosure means includes a vent in the electrode portion and between said spaced electrode means, and a plurality of sidewall vents in the outer end of the arc-elongating portion.

25. An overvoltage gap protective apparatus, comprising an enclosure means defining a spark gap chamber having an electrode portion and an arc-elongating portion extending outwardly from the electrode portion, said chamber having a roughened inner surface at least within said arc-elongating portion.

I FORM F'O-IOSO( UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,577,032 Dated May 4, 1971 lnventofls) Darrel D. McStrack and Lawrence M. Barrage It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 4, line 60 Cancel "of" second occurrence, and

substitute therefor or Col 5, line 63 Cancel "prevent" and substitute there for prevents Col 6, line 32 Cancel "arch" and substitute therefo:

--- arc Col 7 line 27 After "the" first occurrence, inseri high Col. 7 line 72 Cancel "has" and substitute therefor had Col. 8, line 46 Cancel "arch" and substitute therefo1 arc Col. 10, line 23 Cancel "3 3/4" and substitute therefc 3/4 Col 10, line 27 After '1 1/4" cancel "C" Col 10, line 68 After "coil" insert consisting Col ll line 2 After "and" insert said Col ll line 54 Cancel "elongation" and substitute elongating Signed and sealed this 2nd day of November 1 971 (SEAL) Attest:

ROBERT GOTTSCHALK Ac ting Commissioner of Patent 

1. An overvoltage gap protective apparatus, comprising an enclosure means defining a spark gap chamber having an electrode portion and an arc-elongating portion extending outwardly from the electrode portion with a generally continuously reducing cross section to an outer terminal portion, vent means in said enclosure means connected to said terminal portion of the spark gap chamber, spaced electrode means in the electrode portion of said chamber and having terminal means for connecting said electrode means in a circuit to be protected, said spaced electrode means having an arc established therebetween in response to a selected applied voltage, and a magnetic means coupled to said enclosure means to elongate the arc into the arcelongating portion of said chamber.
 2. The overvoltage protective apparatus of claim 1, wherein said magnetic means is an electromagnetic means connected in circuit with said electrode means. 3 The spark gap protective apparatus of claim 1, wherein said magnetic means is an electromagnetic means connected in series with said electrode means.
 4. The spark gap protection apparatus of claim 1 having a second similar enclosure means and a second electrode means in said second enclosure means, and said magnetic means is a common electromagnetic means encircling both of said enclosure means and connected in series between said first and second electrode means.
 5. The spark gap protective apparatus of claim 1, wherein said magnetic means is an electromagnetic coil of a single layer wound about the enclosure means and connected in series with the electrode means.
 6. The overvoltage protective apparatus of claim 1, wherein said enclosure means includes a pair of stacked housings having complementing cavities defining said chamber, the bAses of the cavities including parallel planar surfaces along corresponding sides thereof and having correspondingly tapered wall extending laterally from the planar surfaces and toward each other, and vent means connected to the chamber at the outer end portions of the tapered walls.
 7. The overvoltage protective apparatus of claim 1, wherein said enclosure means includes a pair of stacked housings having complementing cavities defining said chamber, the bases of the cavities including parallel planar surfaces along corresponding sides thereof and having correspondingly tapered roughened walls extending laterally from the planar surfaces and toward each other, said stacked housings being formed of a semiconductor material, a conductive adhesive between said complementing surfaces to interconnect the housings and provide a current path therebetween, a sealing means secured to the outer junction of the housings to seal said chamber, and said vent means being connected to the chamber at the outer end of the tapered roughened wall.
 8. The overvoltage protective apparatus of claim 7, wherein the housings are formed of a nonvitrified material.
 9. An overvoltage protective apparatus comprising an enclosure means including three stacked housings, said housings having complementing cavities defining a pair of arc chambers, the opposite faces of the intermediate housing having similar cavities each including a planar wall along one side thereof and having a tapered wall extending laterally from the planar wall, said cavities having the planar walls on diametrically opposite sides of the housing, the end housings being formed with similarly contoured cavities aligned with the adjacent cavity of intermediate housing to define a pair of spark gap chambers having an electrode portion and an arc elongating portion extending outwardly from the electrode portion with a generally continuously reducing cross section, electrode means including spaced electrodes having a rodlike terminal and support members and means securing an electrode to each of the planar walls with the terminal and support members extending therethrough to locate a pair of spaced electrodes in each arc chamber, the terminal members secured to the intermediate wall terminating in a recess in the tapered wall, means secured within each of the recesses to seal the recesses and define a continuous uninterrupted tapered wall, and a magnetic means coupled to said enclosure means to elongate the arcs into the arc elongating portions of said chambers.
 10. An overvoltage protective apparatus comprising enclosure means including three stacked housings having complementing cavities defining a pair of arc chambers, the opposite faces of the intermediate housing having similar cavities each including a planar wall along one side thereof and having a tapered wall extending laterally from the planar wall, said cavities having the planar walls on diametrically opposite sides of the housing, the end housings being formed with similarly contoured cavities aligned with the adjacent cavity of the intermediate housing to define an electrode portion and an arc elongation portion extending outwardly from the electrode portion, electrode means including electrodes having a rodlike terminal and support members, and means securing an electrode to each of the planar walls with the terminal and support members extending therethrough to locate a pair of spaced electrodes in each arc chamber, said spaced electrodes having an arc established therebetween in response to a selected applied voltage, and an electromagnetic coil means as a single coil tightly wound about the intermediate housing to elongate the arc into the arc-elongating portions of said chambers, and an insulating adhesive covering said coil and attaching said coil to said intermediate housing.
 11. The overvoltage protective apparatus of claim 10, wherein all housings are formed of semiconductive material.
 12. The overvoltage protective apparatus of claim 10, wherein all housings are formed of nonvitrified material.
 13. The overvoltage protective apparatus of claim 1, wherein said enclosure means includes a plurality of stacked housings having complementing cavities defining arc chambers, the opposite faces of an intermediate housing having similar cavities each including a planar wall along one side thereof and having a tapered wall extending laterally from the planar wall, said cavities having the planar walls on diametrically opposite sides of the housing, the adjacent end housings having being formed with similarly contoured cavities aligned with the adjacent cavity of intermediate housing, means securing an electrode to each of the planar walls, and an electromagnetic coil means tightly wound in a single layer construction about the housings, and an insulating adhesive covering said coil means and attaching said coil means to said housings.
 14. The overvoltage protective apparatus of claim 13, wherein the housings having the coil means wound thereon are of nonvitrified material and all other housings are of a semiconducting material.
 15. The overvoltage protective apparatus of claim 1, wherein said enclosure means includes a pair of stacked housings having complementing cavities defining said arc chamber, the bases of the cavities including parallel planar surfaces along corresponding sides thereof and having correspondingly tapered wall extending laterally from the planar surfaces and toward each other, and terminating in a circular sidewall of a selected minimum depth at the outermost ends of said tapered walls, a vent opening extending along a tangent line through the outermost ends of said tapered walls.
 16. An overvoltage protective apparatus comprising an enclosure means defining a spark gap chamber having an electrode portion and an arc-elongating portion extending outwardly from the electrode portion with a generally continuously reducing cross section, spaced electrode means in the electrode portion of said chamber and having terminal means for connecting said electrode means in a circuit to be protected, said spaced electrode means having an arc established therebetween in response to a selected applied voltage, said enclosure means including a second arc chamber having a second pair of electrode means and a third arc chamber housing a third pair of electrode means, said three chambers being arranged in superimposed stacked relation, conducting means connecting one of said first pair of electrode means to one of said second pair and the second of said second pair to a first of said third pair to connect the electrode gaps in series, and electromagnetic means coupled to said enclosure means to elongate the arc into the arc elongating portion of said chambers and connected in parallel with the second pair of electrode means.
 17. The overvoltage protective apparatus of claim 16, wherein each of said arc chambers having a corresponding contour including an electrode portion to one side of the chamber with tapered walls extending outwardly from the electrode portion to define a gap horn, said outer chambers being oppositely axially rotated with respect to the central chamber with the pair of electrode means in the central chamber aligned with one of each of the electrode means in each of said outer chambers.
 18. The overvoltage protective apparatus of claim 1, wherein said enclosure means includes a pair of stacked housings having complementing cavities defining said chamber, the bases of the cavities including parallel planar surfaces along corresponding sides thereof and having corresponding tapered wall extending laterally from the planar surfaces and toward each other, said vent means being connected to the chamber, said electrode means having terminal members extending outwardly through said planar surfaces and supporting a platelike electrode within the chamber, the sidewall of each housing including an inner projection complementing the electrode adjacent the tapered walls, and conductive anchor members coNnected to the corresponding electrodes and extending outwardly along the corresponding projection to the sidewall of the chamber adjacent the tapered walls.
 19. The overvoltage protective apparatus of claim 1, wherein said enclosure means includes a plurality of stacked housings with the mating surfaces having complementing contoured cavities having circular sidewalls and defining a series of arc chambers, the bases of the cavities including parallel planar surfaces along corresponding sides thereof defining said electrode portion of each arc chamber and having corresponding tapered surfaces extending laterally from the planar surfaces and toward each other to define the arc-elongating portions and terminating in axially spaced relation whereby said arc-elongating portions have a continuous sidewall vent passageway defining said vent means connected to the chamber outer ends of said arc-elongating portions, said electrode means having terminal members extending outwardly through said planar surfaces and each supporting a platelike electrode within the corresponding chamber to locate a pair of electrodes in spaced relation within each electrode portion of said chambers, the sidewall of each housing including an inner projection complementing the electrode adjacent the tapered walls, conductive anchor members connected to the corresponding electrodes and extending outwardly along the corresponding projection to the sidewall of the chamber adjacent the tapered walls, contact plates secured to the opposite ends of said stacked housing and connected to a terminal member extending outwardly from the immediately adjacent housing, said magnetic means including an electromagnetic coil encircling said housings and connected in series with at least one of said pair of electrodes.
 20. The overvoltage protective apparatus of claim 19 having at least three similar arc chambers each having the pair of electrodes similarly located therein, chambers being angularly oriented with the electrodes in the outer chambers being angularly spaced in accordance with the spacing of each pair of electrodes, and wherein the electrodes in the central chamber are each aligned with one of the electrodes in the outer chambers and connected thereto by a common terminal member, and said electromagnetic means is connected in parallel with the pair of electrode means in said central chamber.
 21. The overvoltage protective apparatus of claim 19 wherein said enclosure means includes a pair of adjacent similar arc chambers oppositely angularly oriented to locate the electrodes to opposite sides of the enclosure means, and said electromagnetic coil is a single layer coil tightly wound about the central portion of the enclosure means between said adjacent similar arc chambers, and said coil being connected in series between said pairs of electrodes.
 22. An overvoltage spark gap protective apparatus, comprising an enclosure means defining a spark gap chamber having an electrode portion and an arc elongating portion has a maximum depth at the electrode portion, and extends outwardly therefrom with a continuously reduced depth to a terminal portion having a selected depth to establish a continuous sidewall in said arc-elongating portion. 23 The overvoltage gap protective apparatus of claim 22 wherein said spark gap chamber includes vent means to discharge gas generated within said chamber, said vent means being located to discharge said gases in a di-ionized state.
 24. The overvoltage gap protective apparatus of claim 22 wherein said enclosure means includes a vent in the electrode portion and between said spaced electrode means, and a plurality of sidewall vents in the outer end of the arc-elongating portion.
 25. An overvoltage gap protective apparatus, comprising an enclosure means defining a spark gap chamber having an electrode portion and an arc-elongating portion extending outwardly from the electrode portion, said chamber having a roughened inner surface at least within said arc-elongating portion. 